> Imagine for a moment that you want to advertise your new iPhone app on Facebook.

i don't think that's what you're describing here.

> It’s important to know if someone clicks an ad and downloads your app. That’s called a Conversion. It’s used to measure the effectiveness of the ad campaign and the value you gain from spending money on an ad.

i think you've conflated A/B testing with user-profiling. you will obviously know if a campaign has been "effective", if sales increase. you absolutely do not need to uniquely identify a user to do this (which is what happens today).

> That’s unique ID is what Apple wants to remove,

as stated, apple don't want to remove it, merely make it opt-in. and if the end user isn't opting in, they probably don't want it.